Strategies for the in situ immobilization of uranium aim to reduce oxidized and mobile U(VI) to relatively insoluble U(IV), a process long thought to result in the precipitation of the U(IV) mineral uraninite [UO2(s)]. However, recent research indicates that non-crystalline products named monomeric U(IV) may form during the in situ reduction of U(VI)1,2. These products may form via uranium reduction by microbes, by Fe(II) minerals of biogenic origin3, and in biostimulated natural sediments4. Monomeric U(IV) has garnered recent study because it is likely to be more susceptible to reoxidation and remobilization in the environment than uraninite. To test its stability and possible transformations, two types of materials containing monomeric U(IV) were fixed in agarose gel pucks: (1) monomeric U(IV) produced via U(VI) reduction by Shewanella sp. CO-9, and (2) monomeric U(IV) produced by the reduction of U(VI) by phosphate-treated nanoparticulate magnetite of biogenic origin. These gel pucks were deployed in two groundwater wells at the Rifle, Colorado IFRC site for rec overy after 1, 2, and 3 months. Digestions of the gels reveal that uranium is lost more rapidly from gels containing monomeric U(IV) than those containing biogenic uraninite. The monomeric U(IV) associated with biomass is more stable than that associated with magnetite. Uranium LIII-edge X-ray absorption spectroscopy (XAS) data point to a relative enrichment of uraninite in the gels as monomeric U(IV) species are selectively removed by the groundwater over time. Our results provide the first direct evidence of the in situ instability of monomeric U(IV) species relative to uraninite. Thus, the identification and quantification of these species at field remediation sites is likely to be critical in devising accurate uranium transport and fate models. [1] Bernier-Latmani et al. (2010) Environ. Sci. Technol.44, 9456-9462. [2] Fletcher et al. (2010) Environ. Sci. Technol.44, 4705-4709. [3] Veeramani et al. (2011) Geochim. Cosmochim. Acta, 75, 2512-2528. [4] Sharp et al. (2011) Geochim. Cosmochim. Acta, 75, 6497-6510.